Compaction of porous ices rich in water by swift heavy ions

Porous water ice and water ice mixtures H₂O:X (X=CO, CO₂ and CH₄) produced at 15K, with film thicknesses in the 0.5-1μm range, were irradiated by swift ions and monitored by mid-infrared spectroscopy (FTIR). The analysis of the evolution of the pure water ice infrared absorption on ion beam dose reveals a strong correlation among three quantities: (i) the absorbance of the most intense band (3250cm^-1), (ii) the wavelength of the maximum absorbance of this band and (iii) the absorbance of the OH-dangling bonds. This correlation is interpreted as indications of the water ice compaction by irradiation: as the beam fluence increases, the ice porosity decreases, the dangling bond peaks collapse and the area and position of the 3250cm^-1 band vary exponentially, all of them evolving with the same compaction cross section (σ_c). The linear dependence σ_c∝S_e (S_e being the electronic stopping power) is observed for both pure and mixed water ices, confirming previous results. We suggests that the infrared absorption A-value varies with dose as (1-ζe^-D/D₀) during the compaction process (D₀=0.2eV/molec being the effective energy density to eliminate the OH-db, and ζ is a parameter characterizing the porosity). These findings may be used as a diagnostic tool to probe the morphology of water ices occurring in the outer Solar System and in the ISM.